This is a short but true story about a trouble shooting experience I encountered recently. This was originally wrote to be submitted to a trade publication as a guest article in their "Trouble Shooting section" so it was intended to be read by professionals. Hopefully it will make some sense to those of you reading it.
When Getting Shocked Was A Good Thing
It’s never a good idea to deliberately or accidently get connected to a live circuit but the following story shows how by me getting a shock may have saved someone else’s life.
I was called to repair a ground mounted 400 watt high pressure sodium flood light fixture at a commercial location. The customer reported to me that the fixture is staying on all the time. Sounds easy enough check the photo eye! Sure enough the photo eye had water in it and that's never good so I disconnected it and strolled to my truck for a replacement.
After installing the new photo eye and verifying it was functioning correctly I put the cover back on the box and in the process I came into contact with both the box and the fixture at the same time and I received a shock. To myself I thought “Where did that come from?” I pulled out the volt meter and connected from the fixture to the metal FS box and the meter showed 240 volts. Since the supply voltage was 120 volts how was that possible since there was only one circuit in the box. No question about it, we need to verify the grounding path and examine the entire circuit.
The original installation was done using a malleable iron FS style box with two ¾” rigid conduits entering the lower hubs. A cord exited the box through a upper hub and feed out to the fixture. The circuit feeding the FS box was a 12-2 UF cable so the ¾” conduits were for support only. The light fixture was yolk mounted on a small concrete slab less than a foot away from the FS box.
Beyond the grounding issue my first thought was that one of the tap wires must have been touching the ungrounded fixture case. I opened the fixture and checked all the wiring leads and they were all insulated. I also noted that the ballast had been replaced somewhat recently. Further investigation revealed that the UF cable was not grounded at the supply box up near the building. I connected the ground wire and instantly noticed smoke starting to pour out of the back of the light fixture and quickly shut it down.
I went back to the fixture and removed the ballast and realized that the installer who replaced the ballast did not do a very good job. In the process of mounting the ballast he had pushed the ballast into a mounting screw causing a section of the primary winding to ground out to the fixture case. Since it was a multi tap ballast now I know where the 240 volts came from.
I insulated the winding, remounted the ballast verified the grounding path and brought the fixture back on and all is functioning correctly. It was lucky that the moist earth provided enough conductivity that I received a shock during the servicing of the fixture that tipped me off to another deeper and very dangerous problem that otherwise would have gone unnoticed.
Looking at the image below the arrow points where the mounting screw damaged the primary winding insulation and caused it to energize the fixture case.